This invention is directed to an on-off valve and more particularly to an on-off valve especially useful in a fuel injection system for an internal combustion engine.
A known fuel injection system for internal combustion engines is is disclosed in DE 44 01 074 A1. This system has a low-pressure pump that aspirates fuel from a tank and delivers it to a high-pressure pump. An on-off valve with a compression spring-loaded sleeve-shaped valve piston is disposed in the connecting line between the two pumps. This valve piston has a throttle bore in its bottom, via which the low-pressure pump continuously communicates with a camshaft chamber of the high-pressure pump, which is embodied as a radial piston pump. The camshaft chamber is in turn connected to the fuel tank.
In this system, the valve piston can be moved by the pressure of the fuel delivered by the low-pressure pump counter to the force of the compression spring. When a pressure threshold is exceeded, the valve piston opens a line connection to pump elements of the high-pressure pump.
The on-off valve has several functions: on the one hand, the low-pressure part of the fuel injection system can be ventilated on the way via the throttle bore and the camshaft chamber. On the other hand, fuel serving as a lubricant is supplied to the camshaft chamber via the throttle bore. Moreover, in the event of a defect in the high-pressure part of the system, the on-off valve is intended to function as a shut-off valve in order to protect the engine. With this known embodiment, however, it is disadvantageous that the fuel flow not withdrawn by the high-pressure pump results in an uneconomical operation of the low-pressure pump.
The on-off valve according to the invention, has the advantage over the above prior art valve that outside the direct line connection between the low-pressure pump and the high-pressure pump, in addition to the ventilation of the low-pressure part of the fuel injection system and the dimensioning of the lubricant flow to camshaft of the high-pressure pump, it also assures that the fuel volume flow not withdrawn by the high-pressure pump is diverted directly to the suction side of the low-pressure pump. In so doing, the on-off valve controls two fuel circuits, namely the circuit used to lubricate and cool the high-pressure pump and the circuit used to return the diverted fuel, wherein the first circuit remains largely uninfluenced when the second circuit is switched on. Moreover, the return of the diverted fuel directly to the suction side of the low-pressure pump increases the delivery capacity of this pump since a preliminary filter of the system required for filtering the fuel can be bypassed.
The valve of the present invention is advantageous because several valve functions can be performed by a single movable valve member. In addition, the valve can be completely preassembled and tested as a standard component. Due to its disposition in the housing of the high-pressure pump, the installation of the valve into the system involves little expense, in particular, only one seal in relation to the outside is required since the screw thread hydraulically separates the connections of the pump housing to the camshaft chamber and the suction side of the low-pressure pump from each other to a sufficient degree. As a result, the on-off valve can be produced with a relatively short valve housing.